Previous work established that the flow cytometric dihydrorhodamine (DHR)-based assay for evaluation of oxidative burst is a sensitive method for evaluating phagocytes in patients with possible chronic granulomatous disease (CGD). It has also been useful as an accurate test for screening possible X-linked CGD carriers and as a preliminary screen for the genotype of the disease. The method has been optimized and is sensitive to one normal cell in 10,000 or fewer abnormal cells. This project previously demonstrated the capacity to follow normal granulocytes after allogenic leukocyte transfusion in patients with CGD, and this assay has proven to be reliable in establishing duration of transfused cell survival and all-sensitization. The current phase of gene therapy of CGD continues to be directed at patients with X-linked CGD. The DHR assay is being applied as a critical method for establishing the presence of gene-corrected granulocytes in the circulation and for monitoring the duration of production of the gene-corrected cells in the treated patients.